1. Field of the Invention
The present invention relates to vapor recovery systems for preventing the escape of chemical vapor, including hydrocarbon vapors to the atmosphere during the transfer of organic liquid, e.g. gasoline. In particular, the invention relates to recovery of hydrocarbon vapors collected upon the loading and unloading of hydrocarbon transportation vessels such as tank trucks, barges and ships. More particularly, the invention relates to an improvement in the operation of absorber type vapor recovery systems in which the vapors are absorbed in a liquid.
2. Related Art
The art of absorbing vapors in a liquid is old and well known. Generally the vapors are contacted in counter-current flow with the absorbing medium in an absorber column containing liquid-vapor contact structure. The contact structure may be inert packing or special trays of various varieties. Usually the vapors enter at the bottom of the column and the absorbing medium at the top.
The absorber liquid medium entering the top of the column is most generally referred to as "lean oil", especially in hydrocarbon absorptions, to distinguish it from the liquid leaving the bottom of the column which is called "rich oil" because it is saturated with the absorbed vapors. Typically the lean oil is chilled before introduction to the column to promote condensation as well as absorption of the vapors.
The rich oil from the bottom of the column may simply be combined with the lean oil fed to the column until the lean oil has become saturated with the vapors. At that point the "lean oil" must be replaced or "regenerated" by removal of the absorbed vapors, as by fractional distillation, since it is by definition now rich oil. Such a system is described in Environmental Protection Agency Report EPA/450/3-76-038a entitled "Background Information on Hydrocarbon Emissions From Marine Terminal Operations, Vol. I: Discussion", at pages 139-146.
Another common operation is to continuously "regenerate" the rich oil to lean oil by fractional distillation in a stripping column or stripper while the absorption operation is progressing. The lean oil from the stripper is recycled directly back to the absorber. This system is disclosed in U.S. Pat. No. 3,981,156.
The vapors recovered from the distillation may be liquified and stored for blending into a useful product. Alternatively, the liquified hydrocarbons may simply be returned to the vessel from which they came. The alternative disposition of the recovered vapors is disclosed in the above cited U.S. Pat. Nos. 2,849,150 and 2,765,872.
The vapor recovery systems operated at loading and unloading terminals only have to operate during actual loading and unloading of vessels. Quantities and composition of vapors produced vary with seasonal temperature, size of the vessel and the volatility of the product being loaded or unloaded. Due to the cyclical nature of the operations, one reference, U.S. Pat. No. 2,849,150 includes a control for actuating the vapor recovery system when enough vapors have been collected for efficient operation.
In a typical integrated system the energy requirement for the fractional distillation section is at least twice that for the absorption. During start up of the stripper, all of the equipment must be heated and the equilibrium temperature of the system reached. In prior integrated systems this must occur every time a vessel is loaded or unloaded.
One alternative to stripping the rich oil is to use the product being loaded as the lean oil and returning the rich oil to the storage tank as shown in U.S. Pat. No. 3,714,790. However, as shown in U.S. Pat. No. 3,714,790, the lean oil is chilled to about 5.degree. to 15.degree. F. and a reservoir of the chilled liquid is maintained for start up. Because of the temperatures and pressures used therein, the vapor recovery is mainly by condensation. Additionally because of the pressures the compressed vapors must be saturated to prevent explosive concentrations and promote rapid condensation.